Implantable medical devices are available for treating cardiac arrhythmias by delivering anti-tachycardia pacing therapies and electrical shock therapies for cardioverting or defibrillating the heart. Such a device, commonly known as an implantable cardioverter defibrillator or “ICD”, conventionally senses a patient's heart rate and classifies the rate according to a number of heart rate zones in order to detect episodes of tachycardia or fibrillation. Atrial and ventricular arrhythmias are typically detected by initially identifying a fast atrial and/or ventricular rate based on measuring the time intervals between sensed atrial depolarizations or “P-waves” and/or sensed ventricular depolarizations or “R-waves”.
Typically a number of predefined rate zones are defined according to programmable detection interval ranges for detecting slow tachycardia, fast tachycardia and fibrillation. Sensed event intervals falling into defined detection interval ranges are counted to provide a count of tachycardia intervals. A programmable number of intervals to detect (NID) defines the number of tachycardia intervals occurring consecutively or out of a given number of preceding event intervals that are required to detect tachycardia. A separately programmed NID may be defined for detecting slow and fast tachycardia and fibrillation.
Once a tachycardia is detected based on sensed P-wave or R-wave intervals, the morphology of the sensed depolarization signals may be used in discriminating heart rhythms to improve the sensitivity and specificity of arrhythmia detection methods. However, the sensitivity and specificity of such rate- or interval-based arrhythmia detection methods are limited to the reliability of sense amplifiers in accurately sensing P-waves and/or R-waves and by the selection of the rate zone thresholds for tachycardia detection. Interval-based arrhythmia detection schemes that rely primarily on P-wave and R-wave sensing are subject to limitations due to oversensing and undersensing of depolarization signals, which can result in either overestimating or underestimating the actual heart rate. Inadequately programmed rate zone thresholds can also cause over- or under-detection of tachycardias that might be responsive to ICD therapy.